Phytotoxins produced by Streptomyces scabies were suspected as the causal agent of potato scab in 1926 [Fellow H., 1926; King et al. 2009]. The phytotoxins were isolated from S. scabies on immature potato tubers. In 1989 the phytotoxins were found to contain a class of compounds containing 4-nitro-L-tryptophan and phenylalanine linked in an L,L-configured cyclodipeptide, named as thaxtomins [King et al., 1989]. Thaxtomin A is the major metabolite from S. scabies. Subsequently, thaxtomins were also found from other Streptomyces species such as S. acidiscabies, S. turgidiscabies, S. europaeiscabiei, S. niveiscabiei and S. ipomoeae [King et al., 1994 and 2009]. Over 11 analogs of thaxtomins have been purified from these microorganisms.
Thaxtomin A has been found to cause hypertrophy of plant cells at nanomolar amounts and cell death at concentrations similar to those found in scab lesions on field infected potato tubers [Lawrence et al., 1990]. Structural-activity relationship (SAR) studies revealed that the presence of the nitro group at 4-position of indole ring of the tryptophan moiety and an L,L-configuration of the diketopiperazine ring are specific for phytotoxicity [King et al., 1989 & 1992]. Other studies relating to mode of action have suggested that thaxtomins may, like dichlobenil and isoxaben, inhibit the synthesis of cellulose [King et al., 2001, Schneegurt et al., 1994; Scheible et al., 2003]. Consequently, thaxtomins have been investigated as herbicides. U.S. Pat. No. 7,989,393 to Kang et. al. discloses methods for treating or controlling algae using one or more thaxtomins. U.S. Patent Publication 2010/0167930 A1 shows a process using thaxtomin and thaxtomin containing compositions for controlling the germination and growth of weeds in cereal, turf, timothy grass and pasture grass cultures. WO 2010/121079 A2 shows the use of thaxtomin for controlling the germination and growth of broadleaf, algae, sedge and grass weeds, particularly in rice growing systems and/or aquatic based weeds.
During identification and verification of the structure of thaxtomin A, 4-nitrotryptophan was identified as the precise moiety of the structure of thaxtomin A [King et al., 1992]. Subsequently, 4-nitrotryptophan and N-acetyltryptophan were proposed as possible intermediates for thaxtomin A biosynthesis [King et al., 1995; King et al., 2003]. Ultimately, it was verified that 4-nitrotryptophan is a substrate for the non-ribosomal peptide synthetase TxTB in the thaxtomin A biosynthetic pathway [Johnson et al., 2009]. The addition of 4-nitrotryptophan in fermentation broth has been shown to enhance the yield of thaxtomin A [Johnson et al., 2009].
Due to difficulties of enhancing the yield of thaxtomin A through wild strains of Streptomyces species, it is crucial to feed these microorganisms with 4-nitrotryptophan during fermentation. However, this compound is not commercially available. The synthesis of 4-nitrotryptophan has been reported in numerous publications. For example, 4-nitrotryptophan was prepared by tryptophan nitration with nitric acid and acetic acid [King et al., 1992 & 1995] and synthesis of 4-nitrotryptophan derivatives from nitrogramines was also reported [King et al., 2009]. However, there were some shortcomings for such reported synthesis routes. For example, nitration with nitric acid and acetic acid is not selective and results in the formation of numerous reaction products requiring separation procedures. In addition, the 4-nitrotryptophan in thaxtomin A should be in the L configuration, not a D,L racemic mixture synthesized from nitrogramines. Although racemic 4-nitro-D,L-tryptophan also could enhance the yield of thaxtomin A, it is still questionable whether the microorganisms can utilize the 4-nitro-D-tryptophan optical isomer. Therefore, it is important to obtain a reliable synthetic method to synthesize 4-nitro-L-tryptophan.
Thaxtomin produced by fermentation is relatively expensive in comparison to those other herbicides having a similar mode of action but which can be produced by synthetic methods. This is in part because of low yields in the fermentation processes. It is desirable to use a synthetic approach to obtain this compound. A number of methods have been reported in the literature for the synthesis of thaxtomins. The earliest reported method synthesizes thaxtomin A analog without the nitro group on the 4 position of tryptophan in a racemic synthesis starting with 1,4-diacetyl 2,5-piperazinedione [Gelin et al., 1993]. That five step method results in a total yield of about 6.3%. The second reported method shows the synthesis of thaxtomin C in two steps, beginning with the condensation of N-methyl-L-4-nitrotryptophan methyl ester and t-Boc-L-phenylalanine to give a dipeptide methyl ester which is then cyclized to form the thaxtomin analogue [King 1997]. The third method starting from fermented thaxtomin A synthesizes thaxtomin A alkyl ethers for an SAR study [Krasnoff et al., 2005]. The last reported method shows the synthesis of thaxtomin C and thaxtomin D analogues with a 2,5-diketopiperazine core and L-phenylalanine with an apparent racemization in the described procedure [Molesworth et al., 2010]. Molesworth's approach built upon the piperazinedione core of glycine anhydride and the use of aldol condensation chemistry. Additionally, the patent literature shows synthetic methods which use 2,5-diketopiperazine as a core to produce compounds with herbicidal properties. For example, each of U.S. Patent Publication No. 2010/0152047 A1, and each of EP 2 054 394 B1 (U.S. 2010/0173777 A1) and EP 1 971 581 B1 (U.S. 2009/0137396 A1) propose 2,5-diketopiperine derivatives as herbicides. Clearly, an efficient synthetic approach for thaxtomins such as thaxtomin A with proper stereo specificity and intermediates therefore is necessary to provide sufficient quantities for herbicidal uses.